Field of the Invention
The present invention relates to a method of controlling a hydraulic system which is applied to a ram type extrusion device or the like used to extrude a metal material such as aluminum, and to a hydraulic system.
Description of the Related Art
As an apparatus for extruding a metal material such as aluminum, a ram type extrusion device has been known which includes a tubular container receiving an unwrought metal (billet) such as aluminum, a die attached to an exit side of the container, and a ram cylinder unit connected to an entrance side of the container.
This ram type extrusion device is known as a type of hydraulic apparatus that receives, for example, an unwrought aluminum in the container, that presses a ram of the ram cylinder unit into the container from the rear side of the container, that pushes the billet to the die under high temperature and high pressure, and that processes the billet to a product of a desired shape by extrusion through a die hole formed in the die.
In this extrusion device, when the extruding speed of the billet passing through the die is higher than necessary, the surface of an extruded product of aluminum may be roughened or cracks may be formed, whereby the extruded product may become defective. Accordingly, from the viewpoint of productivity, it is preferable that the extruding speed of the billet be set as high as possible without causing a defective product and that the extruding speed of the billet be made to reach a target speed fast and become stabilized.
Therefore, when operating the extrusion device, it is possible to achieve uniformization of the extruding speed of the billet by setting the target value of the extruding speed and controlling the pressure or the speed of the ram causing the billet to move. However, it is not easy to control the extruding speed due to the variation in extrusion resistance between the initial period of extrusion and the later period of extrusion based on the amount of billet remaining, the influence of conditions unique to the extrusion device, and the like. Accordingly, control of the extruding force or the extruding speed of the ram cylinder unit is an important factor in operating the extrusion device.
As an example of a control technique of the extrusion device, a PID control method is known as a control method of a ram cylinder unit.
Japanese Unexamined Patent Application, First Publication No. 2000-222002 discloses a control parameter determining device that includes a storage part storing presence time for each of plural operating value sections which are partitioned depending on the magnitudes in an extended PID control technique, means for adding the presence time to the storage parts of an operating value section corresponding to the acquired operating value to be controlled and an operating value section smaller than the operating value section, and determination means for determining initial parameters relevant to the initial operating value based on the largest operating value section of the operating value sections of which the presence time stored in the storage part is larger than a threshold value.
In the method of controlling the ram cylinder unit using the PID control technique, the speed of the ram of the ram cylinder unit is controlled to be a target value lower by a predetermined margin than the critical point between a non-defective product and a defective product. In the PID control, the speed is controlled to reach the target value as fast as possible in the transition period which is the initial period of control. In a steady state after the speed reaches the target value, the speed is controlled to maintain the target value without changing the control value.
However, in the PID control, particularly, in the transition period, a proportional operation of controlling an input gain of the ram cylinder unit is performed based on the difference between the target value and the actual value of the control value such that the operating value becomes larger when the difference becomes larger and the operating value becomes smaller when the difference becomes smaller.
For example, in the PID control, the P operation is to acquire the difference between the target speed and the present speed of the ram and to set the output to be large so as to obtain an output proportional to the difference when the difference between the target speed and the present speed is large. The I operation is to set the output to be large so as to obtain the output proportional to the integral of the difference when it is hard to raise the present speed to the target speed. The D operation is to control the output in the direction in which the rapid variation of the difference is suppressed so as to obtain the output proportional to the differential of the difference between the target speed and the present speed of the ram.
That is, the PID control is a control method such as trial-and-error control in consideration of the difference between the target value and the actual value. Accordingly, when the input gain of the ram cylinder unit is small, there is a problem in that the present speed barely reaches the target speed. When the input gain of the ram cylinder unit is large, there is a problem in that overshoot is caused or a variation in the output value called hunting is caused. There is also a problem in that it is not easy to appropriately match the input gain of the ram cylinder unit.
In the control method described in JP2000-222002A, control having fuzzy control and a learning function combined therein is performed. This method is basically feedback control. Accordingly, when an aluminum material is first extruded under different conditions, the adjustment is difficult. In the second or subsequent control using the same material, a learning effect is exhibited and the accuracy is improved to a certain degree. However, since this is a control method of performing control in a state of trial-and-error in consideration of the state, the speed of the ram barely reaches the target speed. Accordingly, there is a tendency that the problem with overshoot or hunting in addition to the problem with the slow response is not solved.
In the control method described in JP2000-222002A, the conditions in the control of second or subsequent extruding are different from the previous ones, when the billet temperature, the die temperature, and the billet length are changed. Accordingly, there is a problem in that the previous learning effect is not exhibited and the control method conclusively becomes a control method which is in a state of trial-and-error.